This invention relates to tantalum compounds which are catalysts or catalyst precursors for the dimerization of 1-olefins to 1-butenes.
Higher olefins, such as 1-butenes, find application as industrial intermediates. 2,3-Dimethyl-1-butenes, for example, are used as a gasoline additive to improve octane rating and as a starting material to synthesize musk perfumes.
The product of the dimerization of a terminal olefin is dependent on the direction of coupling of the two olefinic units; hence, a mixture of dimeric products is always possible. In order to achieve a high yield of a particular dimeric product, a highly selective catalyst is required. Prior to 1980 few homogeneous catalysts were known for converting a terminal olefin selectively to 1-butene or 2,3-disubstituted-1-butenes at room temperature or above. In addition, the known catalysts readily isomerized the initially formed 1-butenes to the thermodynamically more stable internal olefins. Moreover, some of the catalysts were active with only one olefinic substrate. Certain nickel compounds containing phosphine ligands typify the dimerization catalysts of this period. One example of said nickel compounds is tris(triisopropylphosphine)nickel(O), which in the dimerization of propylene, yields a mixture of dimethylbutenes, methylpentenes and linear hexenes. For a general review of this subject, see B. Bogdanovic, Advances in Organometallic Chemistry, 17 (1979), 105-140; and S. Muthukamari Pillai et al., Chemical Reviews, 86 (1986), 353-399.
More recently a series of tantalum compounds has been disclosed in U.S. Pat. Nos. 4,197,419; 4,231,947; and 4,245,131. It is further disclosed that these tantalum compounds are capable of selectively dimerizing 1-olefins, in general, to 1-butene and 2,3-disubstituted-1-butenes. These tantalum compounds contain a cyclopentadienyl group represented by the formula C.sub.5 H.sub.5-x Me.sub.x, wherein Me is methyl and x is an integer from 0 to 5. The tantalum compounds containing this cyclopentadienyl group and their use as catalysts in the dimerization of 1-olefins are described hereinbelow.
U.S. Pat. No. 4,197,419 discloses tantalum catalyst precursors or catalysts of the formulae ##STR1## where C.sub.5 Me.sub.5 is pentamethylcyclopentadienyl; X is halide or alkoxide; L is an alkene having from 2 to 20 carbon atoms; and R.sup.2 is hydrogen or a C.sub.1-18 alkyl radical. These precursors or catalysts are disclosed to dimerize 1-olefins, such as 1-propylene, 1-pentene and 1-hexene, to 1-butenes selectively. For example, it is taught that propylene is dimerized with greater than 90 percent selectivity to 2,3-dimethyl-1-butene at a rate of about 0.039 min.sup.-1 or about 1 turnover per Ta per hour at 40.degree. C.
U.S. Pat. No. 4,231,947 and U.S. Pat. No. 4,245,131 disclose catalysts or catalyst precursors of the formula Z(R)(R.sup.1).sub.n (R.sup.2)(A).sub.m wherein Z is tantalum or niobium; R is cyclopentadienyl or methyl-substituted cyclopentadienyl having the formula C.sub.5 H.sub.x Me.sub.5-x, wherein x is an integer from 0 to 5, or R is neopentylidene; R.sup.1 is benzyl or neopentyl, n is 0 or 1; R.sup.2 is neopentylidene, benzylidene, tetramethylene or 2,3-dimethyltetramethylene; A is halo including chloro, bromo, iodo and fluoro or a moiety of the formula YR.sup.3 R.sup.4 R.sup.5 wherein Y is a group element including N, P, Sb and Bi, and R.sup.3, R.sup.4 and R.sup.5 can be the same or different and C.sub.1-4 alkyl, aralkyl or aryl; and m is 1 or 2. In the dimerization of propylene by [Ta(C.sub.5 H.sub.5)(CHCMe.sub.3)(Cl).sub.2 ], it is taught that 2,3-dimethyl-1-butene is produced in 93 percent selectivity at a rate of 2 moles per Ta per hour at 45.degree. C.
The preparation of the methyl-substituted cyclopentadienyl group of the aforementioned catalysts is lengthy and costly; for example, the synthesis of lithium pentamethylcyclopentadienide is described in four steps in Scheme 1. Commercially unavailable 2-bromo-2-butene is reacted with lithium, and the lithiated product is condensed with ethyl acetate. The condensation product is cyclized and the resulting pentamethylcyclopentadiene ring compound is reacted with butyl lithium. These reactions give lithium pentamethylcyclopentadienide, the anion of which can be introduced into a tantalum compound. ##STR2##
One skilled in the art can readily appreciate the difficulties and cost of preparing lithium pentamethylcyclopentadienide by the reactions outlined in Scheme 1. Moreover, after the substituted cyclopentadienide group is prepared, the synthesis of the tantalum catalysts containing said cyclopentadienyl group must be accomplished. The prior art patents cited hereinabove teach the synthesis of such catalysts; but, again the synthesis is lengthy and costly.
In view of the prior art it would be desirable to provide a catalyst system which could be prepared easily from commercially available starting materials and which could find use in the selective dimerization of 1-olefins to 1-butenes. Additionally, it would be desirable if the easily prepared catalyst would possess a higher activity in the dimerization of 1-olefins than the homogeneous catalysts known heretofore.